Production of dairy
goats is on the increase in East African countries. As in cows, prevalence of
mastitis in dairy goats appears to be high but studies on the influence of this
disease on milk yield and composition are scarce. This study was, therefore,
carried out to determine the effect of sub-clinical mastitis on milk yield and
composition in dairy goats of Magadu dairy farm in Morogoro, Tanzania. Does were screened for mastitis using the California Mastitis Test (CMT), milk yield was
recorded and 80 ml samples collected for laboratory analyses. A total of 184 quarter
milk yield samples were available for analyses. Milk samples were analysed for
butterfat (BF) by the Gerber method, crude protein (CP) by the Kjeldahl method,
lactose by IDF Standard No.28 method and chloride by titration using silver
nitrate solution. Data were analysed using least squares analysis of variance
based on General Linear models procedures. The fixed effects considered were
CMT score, parity, stage of lactation, sampling occasion and quarter of the
udder. Sub-clinical mastitis had a significant negative effect on quarter milk
yield (P<0.001). Up to 29.4% milk reduction due to mastitis was recorded.
Mastitis had a significant (P<0.05) effect on quarter crude protein, butter
fat and chloride percentages but not on lactose content. There was a tendency
for CP and chloride percentages to increase with increase in CMT scores.
However, the mean chloride value obtained in this study of 0.244 was higher
than expected. Further, mastitis significantly reduced BF content of milk from
6.32 in non-mastitic does to 4.91% among those with highest CMT score. The mean
lactose percentage was 3.83. Sub-clinical mastitis had no significant influence
on lactose content but there was a tendency for a decrease in this component
with increase in severity of mastitis. It is recommended that further studies
involving a bigger number of does with clinical mastitis be carried out to
substantiate the present findings. Economic losses resulting from mastitis in
goats also need to be assessed.

Key
words: Mastitis, milk yield, composition, goats.

INTRODUCTION

Mastitis
is universally recognised as one of the most costly disease in the dairy
industry [1]. The major components of the economic losses caused by mastitis
are: reduced milk yield, veterinary charges, increased labour costs, discarded
milk and reduced longevity of milking goats [2]. High mortalities of kids born
to goats with mastitis make the disease economically significant to the goat
industry [3]. Milk yield in dairy animals can be reduced by as much as 25% and
at times up to 100% [4]. The monetary loss the farmer gets as a result of this disease
due to cows not producing optimally was estimated in Tanzania to be about Tsh.
52,800.00 (about US $42) per cow per year, which was quite high [5].

Reports
on the effect of bovine mastitis on milk composition show consistently lower
lactose and higher chloride contents in quarters with mastitis than in those
without mastitis [5,6]. Changes in fat content as a result of mastitis vary
between sample populations mainly because the variation largely depends on the
severity of mastitis and the magnitude of reduction in milk yield. The effect
of mastitis on crude protein content of milk is not very clear under
sub-clinical conditions although it is well known that at advanced stages, the
trend is an increase in crude protein [7].

The
majority of studies on the influence of mastitis on milk composition have been
conducted in dairy cattle. In Tanzania there is a rapid increase of dairy goats
especially among resource poor smallholder farmers. These goats contribute
substantially towards sustainable supply of milk for children and women. Family
incomes and nutrition have appreciably improved in the areas where dairy goats
have been introduced [8]. However, an earlier study in two locations of Tanzania has clearly shown a high prevalence of sub-clinical mastitis in goats of over 70%
[8]. Such a high incidence calls for the need to evaluate the effect of this
disease on the yield and composition of milk. This is of paramount importance
because there is limited information in goats on the effect of clinical and
sub-clinical mastitis on milk yield and composition [4]. The objective of this
study was therefore to establish the effect of sub-clinical mastitis on milk
yield and composition in dairy goats.

MATERIALS AND METHODS

Study
area and animals

This
study was carried out at Magadu dairy farm. The farm belongs to the Department
of Animal Science and Production of the Sokoine University of Agriculture
(SUA), Morogoro, Tanzania. SUA is situated about 2.5 km south of Morogoro
municipality at an altitude of 550 m above sea level and gets an average annual
rainfall of approximately 880 mm. For most parts of the year temperatures vary
between 27° and 31° C.

The
goats at Magadu dairy farm were first crosses (F1; Norwegian
landrace x Local Tanzanian goats) with 50% Norwegian blood.

Milk
yield and screening for mastitis

Records
on kidding date and parity for individual animals were taken. All lactating
animals were screened and scored for mastitis using the California Mastitis
Test (CMT) as described by Schalm and Noorlander [9]. Following screening, the
CMT score of each udder quarter was included in the individual animal records.
Goats were screened twice at two months interval.

All
quarters screened for mastitis were milked to completion immediately following
screening and in the next milking. The does were milked twice per day at around
6am and 5pm and the volume of milk obtained in each instance was measured in
ml.

Collection
of milk samples

About
80 ml of milk were collected from each of the screened quarter for milk
composition analysis. Milk samples were preserved with potassium dichromate
[10]. Before analyses composite samples were obtained by mixing quarter morning
and evening samples in proportion to the milk yield at each milking.

Analyses
for milkconstituents

Milk
samples from each quarter were analysed for crude protein (CP), butterfat (BF),
lactose and chloride percentages. Total protein was determined by the standard
Kjeldahl method for nitrogen analysis [11]. Nitrogen content was multiplied by
6.38 to obtain the CP percentage. The BF content was determined by the standard
Gerber method [12]. The lactose content was determined according to the method
described by IDF Standard No.28 [13]. The chloride content of the milk samples
was determined by titration using silver nitrate solution [14]. Koestler values
usually increase in cases of mastitis, thus indicating infection. For normal
milk this value is around 2.3. Koestler values (in percent) were computed by
dividing the chloride content by lactose percentage for all CMT scores.

The
regression (covariate) was not included in the model when analysing milk yield
data and effect of milking time was not fitted in the model when analysing milk
components because morning and afternoon samples were mixed proportionately.

RESULTS

Milk
yield

Least
squares means of quarter milk yield per milking for the various factors are
presented in Table 1. The overall mean quarter milk yield was 136.4 ± 3.9 ml.
Sub-clinical mastitis had a significant negative effect on quarter milk yield
(P<0.001). There was an apparent decrease in milk yield with increase in CMT
score. Effect of stage of lactation on milk yield was also significant (P<0.001).
The effects of milking time and sampling occasion were highly significant
(P<0.001) sources of variation. Morning milk yield was 40.4% higher than
afternoon milk.

The
overall mean quarter CP percentage was 4.02 ± 0.08(Table 1). Mastitis had a
significant (P<0.01) effect on quarter CP percentage while parity and stage
of lactation had no significant effect on levels of protein content. Quarters
with highest CMT score had highest CP content.

The
mean quarter BF percentage was 5.32 ± 0.13(Table 2). There was a significant
(P<0.01) reduction in BF content with increasing CMT score. Other factors
that significantly influenced BF content were sampling occasion (P<0.001),
stage of lactation (P<0.05) and milk yield (P<0.001). Butterfat increased
from first (4.30%) to second occasion (6.35%) of sampling and also increased
with stage of lactation from 4.59% in first stage to 6.16% in the last stage.
Further, BF content in quarters with mastitis was lower compared to those with
negative CMT score.

Table 2: Least
squares means (LSM±SE) of butterfat and lactose for various factors

The
overall mean quarter lactose percentage was 3.83 ± 0.08. The least square means
of quarter lactose percent in relation to the various factors are shown in
Table 2. None of the factors studied had a significant effect on quarter
lactose percentage although lactose content tended to decrease with increase in
CMT score.

Table
3 presents the least squares means for quarter chloride percentage for the
various factors under study. The overall mean quarter chloride percentage was
0.244 ± 0.003. The effect of mastitis on quarter chloride percentage was highly
significant (P<0.001) with a tendency for chloride content in milk to
increase with increase in CMT score. Other significant effects on chloride
percent included parity (P<0.001), sampling occasion (P<0.001) and udder
quarter (P<0.05).

A
very high Koestler value of 6.4% was obtained when it was computed using
overall mean percentages of chloride and lactose. Koestler values for negative,
trace, +1, +2 and +3 CMT scores were 5.3, 5.7, 6.7, 5.7 and 8.1, respectively
showing an increase in values with severity of mastitis.

DISCUSSION

Milk
yield

Sub-clinical
mastitis had a significant negative effect on quarter milk yield. Such findings
concur with previous reports in dairy goats [16] and in dairy cows [5,6], which
showed significantly lower milk yield in mastitic quarters than in non-mastitic
quarters. Mastitis is associated with tissue damage that brings about lowered
milk yield or cessation of milk synthesis. The mechanism inhibiting the milk synthetic
activity has been discussed in detail by a number of workers [2, 17].

Milk
composition

The
significant increase in CP content observed at Magadu has also been reported in
severe bovine mastitis [7]. With severe inflammation, the combined effects of
increased seepage of blood derived proteins and increased concentration of
protein due to lowered milk yield may result in increased level of protein. In
cases where CP content does not change, studies have shown that casein is
dramatically reduced while the level of non-casein protein increases resulting
in no change of CP [7]. Contrasting results have been reported by those working
with bovine mastitis who found no significant change in CP with severity of
mastitis [5, 6].

In
this study mastitis significantly affected BF percentage but conflicting
results have been reported on the effect of bovine mastitis on BF content.
While some authors have demonstrated a reduction in BF, others have shown BF
percentage to increase with severity of mastitis [5, 7, 18, 19]. Severity of
mastitis seems to determine the magnitude of the effect. In mild cases of
mastitis, milk yield is reduced without concomitant reduction in BF percentage.
However, with severe inflammation both milk yield and BF concentration are
significantly reduced.

There
was a general tendency for lactose to decrease with increasing CMT score though
this milk component was not significantly affected by sub-clinical mastitis.
Studies with bovine mastitis have shown dramatic reduction in lactose content
with severity of mastitis [5, 6]. Goats are reported to be less susceptible to
mastitis and that partly explains the lesser effect of sub-clinical mastitis on
milk lactose content observed in this study [2]. It is further speculated that
the level of mastitis as indicated by CMT score did not reach a stage of
causing significant reduction in lactose percentage. Further studies with
clinical mastitis are, therefore, needed in order to establish its influence on
lactose content in goat milk.

The
significant increase in chloride percent with severity of mastitis concurs with
earlier reports [5, 6]. The increase in chloride percent with mastitis is due
to the increased permeability of blood vessels, which allows seepage of plasma
into the udder. Even a small amount of blood derived exudates in milk is
reported to result into an elevation in amount of chloride [20].

The
overall percent chloride of 0.244% was higher than results observed in the
literature where chloride percentages of a range of 0.107 to 0.193% have been
reported [21, 22]. The high mean would suggest existence of mastitis in the
flock that was studied as was reported earlier [8].

In
mastitis cases, as lactose decreases it is compensated for by increase in
chlorides in order to maintain osmotic pressure. However, the Koestler values
obtained in this study are much higher compared to that shown by other reviews
of about 2.3% [23]. The main reason for high Koestler values here is that the
chloride content in milk was almost double the expected values as discussed
above.

CONCLUSION
AND RECOMMENDATION

Conclusion

It has been shown that sub-clinical mastitis can reduce
milk yield up to 29.4%.

There was a tendency for CP and chloride percentages to
increase with severity of mastitis, however, the chloride values obtained in
this study were higher than expected.

Mastitis significantly reduced BF content of milk.
Further, sub-clinical mastitis had no significant influence on lactose content
though there was a tendency for this component to decrease with increase in CMT
score.

Recommendation

It is recommended that further studies involving a larger sample of
does and those with clinical mastitis be conducted to substantiate the present
findings and economic losses resulting from mastitis in goats be assessed.

ACKNOWLEDGEMENT

Financial support
from the Norwegian Agency for International Development (NORAD) is gratefully
acknowledged.